The present invention relates to a holding device for studs which have a radially projecting flange section, comprising a collet component which has a clamping section and a hollow insertion section, wherein a stud can be inserted via the insertion section to the clamping section, wherein the clamping section is formed by clamping surfaces which are provided at the free ends of a plurality of clamping arms and which are designed in order to exert a radially inwardly directed clamping force for clamping the stud in place, and wherein a securing device, past which a flange section of the stud can be directed, is arranged on the insertion section and is designed with respect to the clamping section in such a way that a flange section which is directed past the securing device is mounted in a secured manner in a securing position between the securing device and the clamping section.
Such a holding device is known from document DE 296 17 208 U1.
The present invention also relates to a shank-clamping device which can be inserted into a holding device for studs and is designed in order to accommodate a shank section of a stud by means of a shank-locating section and in order to shift the stud from a securing position into a ready position.
Furthermore, the present invention relates to a joining head for joining studs to workpieces, comprising a holding device according to the invention and a shank-clamping device according to the invention, and also relates to a method of feeding a stud to a joining head.
In the field of joining technology, it is known to join studs to the surface of a workpiece. This includes so-called “stud welding”, in which a stud is welded to the surface of a workpiece. Alternative joining techniques include, for example, the adhesive bonding of a stud to the surface of a workpiece.
Such processes are frequently carried out in an automated manner, for example in the motor vehicle industry, where a multiplicity of such studs are joined to the vehicle sheet in order to provide anchors for fastening means, panelling, etc. The automated joining of studs to workpieces as a rule includes the provision of a joining head on a robot. The joining head is in this case connected to a supply unit which provides, for example, the electric welding current and other control signals. Furthermore, it is preferred to feed the studs to the joining head in an automated manner. As a rule, this is effected by means of compressed air through feed tubes. For high cycle times, it is appropriate in this case to feed the studs from the rear, as it were, into a holding device of the joining head. The holding device serves to shift the stud into a defined ready position, starting from which a joining process can be initiated.
The diameter of the feed passages is as a rule slightly larger than the diameter of the flange section in order to make it possible to transport the stud therein with an easy motion. In the holding device, too, the inside diameter of the insertion section is as a rule slightly larger than the outside diameter of the flange section. This may result in a stud fed to the holding device coming into an oblique position in the holding direction and first having to be oriented (centered) again with a welding axis before the stud can be transferred into the ready position. In some situations, it may be the case that this centering does not succeed, the result of which is that the fed stud then has to be ejected from the holding device. In the process, the stud falls downwards in an uncontrolled manner, either onto the floor or also, for example in automobile construction, into the body. The ejected studs then lie distributed on the floor and are swept up and thrown away. Studs left lying in a body may subsequently lead to disturbing noises during driving.
The aim of document DE 296 17 208 U1 is to provide a stud holder in which a stud can be located in a precise position. To this end, a stud holder is proposed which has a plurality of elastic arms which form a first holder. Furthermore, a number of arms are designed to be shorter than the other arms and thus form a second holder. If a flange section of a stud passes this second, holder, it can be fixed between the first and the second holders. In particular, this is intended to provide better centering of the stud.
However, in the case of such holding devices having arms of different length, a situation may occur in which weld or molten pool spatter or contaminants enter the stud holder during the welding operation through the gaps between the arms. In particular cooled weld spatter can be cleaned from the stud holder only with difficulty, and therefore it is often necessary to exchange the stud holder after a certain time. Furthermore, due to weld or molten pool spatter, the stud holder and the loading pin insert can be welded to one another and the loading pin can no longer be displaced, which may lead to a system failure.